1. Field of the Invention
The present invention relates to an improved method for regenerating a catalytic reactor having sulfur deposited on the catalyst. More specifically, it is concerned with a dual temperature process for removing deposited sulfur, particularly in a sulfur recovery plant which employs a low temperature Claus reaction step with a periodic regeneration step, using hot indigenous streams wherein the temperature of the regenerating catalyst bed having once achieved a temperature high enough to promote the hydrolysis of CS.sub.2 and COS is elevated to about 700.degree. F.
2. Description of the Prior Art
Sulfur recovery plants convert hydrogen sulfide to sulfur by a process wherein a portion of the hydrogen sulfide is oxidized to produce a reaction mixture in which hydrogen sulfide and sulfur dioxide are present in a molar ratio of about 2:1, followed generally by two or more catalytic reaction steps in which hydrogen sulfide and sulfur dioxide are converted to sulfur by the Claus reaction. Additional details of the process include cooling the process stream to condense sulfur at appropriate points and reheating of the stream to the desired inlet temperature for each catalytic reaction step. One or more of the initial catalytic steps operate at a temperature which is above the sulfur dewpoint of the reacting gas, as has been practiced for decades. In a recent development to increase the sulfur recovery level, one or more of the final catalytic steps operate at a low termperature where sulfur is strongly adsorbed on the catalyst. This step is referred to as a "low temperature" Claus reaction step. As applied in the so-called Cold Bed Adsorption (CBA) process, typically as described in U.S. Pat. Nos. 3,702,884; 3,758,676; 3,749,762; and 4,035,474, the hydrogen sulfide and sulfur dioxide content of a conventional Claus plant effluent gas stream is decreased by conversion in a Claus type reactor operating at a temperature between about 270.degree. and 300.degree. F. At this lower than usual temperature level, sulfur is deposited on the catalyst which tends to decrease catalyst activity. The flow of the aforesaid dilute stream is periodically switched to a freshly regenerated reactor, and the first reactor in which sulfur has been adsorbed on the catalyst is regenerated by introducing a hot (600.degree. to 650.degree. F.) stream into the bed to vaporize the sulfur which is removed and sent to a condenser. After the catalyst has been regenerated in this fashion, it is cooled by flowing a suitable gas through it at a temperature ranging from about 250.degree. to 300.degree. F. until the catalyst bed itself reaches a temperature level within the aforesaid range. The adsorption cycle of a commercial scale plant typically would be about 18 to 24 hours, while the regeneration would take about 12-18 hours, followed by about 6-12 hours of cooling.
Various methods of accomplishing the overall low temperature Claus process, including the required regeneration and cooling of the catalyst bed, have been proposed. For example, in U.S. Pat. No. 3,702,884, a pair of low temperature reactors is used as an addition to a Claus plant wherein one of the added reactors is operated in the adsorption mode while the other is being regenerated and cooled in a recycle mode. Various arrangements for flow of regeneration effluent gas may be employed by one skilled in the art. The regeneration effluent may flow to a condenser and a blower to recycle the gas back to the appropriate point in the process. In U.S. Pat. No. 3,758,676 and U.S. Pat. No. 3,749,762, processes involving at least three catalytic reactors are disclosed wherein all three reactors are periodically changed from one mode to another, the modes generally comprising a conventional Claus reaction, a low temperature Claus reaction, or a regeneration and cooling mode. In both of these processes, the regeneration and cooling steps are accomplished by altering the relative positions of the reactors in the overall flow as well as controlling the temperature.
In U.S. Pat. No. 4,035,474, a regeneration technique is used wherein the same vessel always functions as the first Claus reactor, maintained at an elevated temperature sufficiently high to hydrolyze the COS and CS.sub.2 present in the Claus furnace effluent. In order to insure that all of the furnace effluent (i.e., all of the COS and CS.sub.2) experiences high temperature hydrolysis conditions, the indigenous stream for regeneration is obtained by withdrawing a portion of the effluent from the intentionally hot first Claus reactor. In this manner, ultimate emissions of sulfur to the atmosphere caused by the presence of COS and CS.sub.2 are minimized. In view of the contemporary emission standards, the COS and CS.sub.2 presence can be critical even though their presence amounts to a relatively small portion of the total sulfur. In fact, all of the above processes, when properly designed and engineered, can achieve as high as 99% removal of total sulfur. The high temperature hydrolysis of COS and CS.sub.2 is an essential element in any sulfur recovery method that is intended to meet contemporary emission standards; however, the use of a hot first Claus reactor, as suggested in U.S. Pat. No. 4,035,474, to process all of the Claus furnace effluent has certain inherent limitations. For example, as is recognized in the art, the primary purpose for the Claus reactor is to catalytically convert H.sub.2 S and SO.sub.2 to elemental sulfur and water. This process involves a reversible exothermic reaction which inherently means that higher temperature shifts the equilibrium concentrations such as to favor the reactants rather than the products, or conversely, lower temperature favors conversion to sulfur and thus higher sulfur recovery. Therefore, the selection of an operating temperature for the first Claus reactor is a tradeoff between the desired decomposition of minor components while still maintaining a high level of conversion of H.sub.2 S and SO.sub.2 to elemental sulfur by the Claus reaction.